Injection molded shaft

ABSTRACT

In a shaft body, a first end in an axial direction of a core part having a crisscross cross section orthogonal to the axial direction is connected to a helical gear via a first connecting part, and the second end in the axial direction of the core part is connected to a worm via a second connecting part. A plurality of first framework parts are formed at regular intervals along a direction of a shaft core in the core part between the first connecting part and the second connecting part. A second framework part extends across the first connecting part, the first framework part, and the core part like a diagonal brace. A third framework part extends across a pair of the first framework parts adjacent to each other and the core part like a diagonal brace. A fourth framework part extends across the second connecting part, the first framework part, and the core part like a diagonal brace.

TECHNICAL FIELD

The present invention relates to an injection molded shaft formed into adesired shape by injection molding, and that can absorb rotation energyby undergoing twisting deformation.

BACKGROUND OF THE INVENTION

Many shafts used as power transmission components for automobiles or thelike are made of metal and are created by a cutting operation, so theproduct cost is high and the weight is heavy.

An injection molded shaft 100 made of synthetic resin in FIG. 10 wasproposed to solve such problems. In the injection molded shaft 100illustrated in FIG. 10, a gear 101 and an inward flange 102 are formedintegrally with one end of a cylindrical shaft body 103 and a rotarytorque is transferred to another rotary component (not illustrated) viathe gear 101 (see JP-A-2003-33947).

However, since the twisting stiffness of the cylindrical shaft body 103is large in the injection molded shaft 100 illustrated in FIG. 10, evenwhen a rotary torque is applied impulsively, the shaft body 103 cannotundergo sufficient twisting deformation and the shock caused by suddenchanges in torque could not be absorbed by twisting deformation of theshaft body 103. Therefore, in the injection molded shaft 100 illustratedin FIG. 10, rotation transmission components such as the gear 101receive a shock caused by sudden changes in torque, possibly breakingrotation transmission components such as the gear 101.

Therefore, the invention provides an injection molded shaft that canabsorb a shock caused by sudden changes in torque by undergoing twistingdeformation of a shaft body when sudden changes in torque are applied.

SUMMARY OF THE INVENTION

As illustrated in FIG. 9, the invention relates to an injection moldedshaft 1 including a first torque applied part 3 formed at one end in anaxial direction, a second torque applied part 4 formed at another end inthe axial direction, and a shaft body 2 connecting the first torqueapplied part 3 to the second torque applied part 4 along a direction ofa shaft core (longitudinal center axis) 17. In the invention, the shaftbody 2 includes a first connecting part 15 formed integrally with thefirst torque applied part 3, a second connecting part 16 formedintegrally with the second torque applied part 4, a core part 18extending from the first connecting part 15 to the second connectingpart 16 along the shaft core (longitudinal center axis) 17, the corepart 18 having a crisscross cross section orthogonal to the axialdirection, and a diagonal-brace-shaped framework part 60 disposed in aportion partitioned by the first connecting part 15, the secondconnecting part 16, and the core part 18, the diagonal-brace-shapedframework part 60 extending across the first connecting part 15, thesecond connecting part 16, and the core part 18 like a diagonal brace.

In addition, as illustrated in FIGS. 1 to 9, the invention relates to aninjection molded shaft 1 including a first torque applied part 3 formedat one end in an axial direction, a second torque applied part 4 formedat another end in the axial direction, and a shaft body 2 connecting thefirst torque applied part 3 to the second torque applied part 4 along adirection of a shaft core (i.e., longitudinal center axis) 17. In theinvention, the shaft body 2 includes at least one framework unit 61. Inaddition, the framework unit 61 includes a core part 18 extending alongthe shaft core (longitudinal center axis) 17, and the core part 18having a crisscross cross section orthogonal to the axial direction. Apair of discoid framework parts 62 and 62 are disposed at one end andanother end along the shaft core (center axis) 17 of the core part 18 soas to face each other, the pair of discoid framework parts 62 and 62having discoid cross sections orthogonal to the axial direction, and adiagonal-brace-shaped framework part 60 disposed in a portionpartitioned by the pair of discoid framework parts 62 and 62 and thecore part 18. The diagonal-brace-shaped framework part 60 extendingacross the pair of discoid framework parts 62 and 62 and the core part18 like a diagonal brace.

In addition, as illustrated in FIGS. 1 to 8, the invention relates to aninjection molded shaft 1 including a first torque applied part 3 formedat one end in an axial direction, a second torque applied part 4 formedat another end in the axial direction, and a shaft body 2 connecting thefirst torque applied part 3 to the second torque applied part 4 along adirection of a shaft core. In the invention, the shaft body 2 includes

-   -   a first connecting part 15 formed integrally with the first        torque applied part 3,    -   a second connecting part 16 formed integrally with the second        torque applied part 4,    -   a core part 18 extending from the first connecting part 15 to        the second connecting part 16 along the shaft core (center axis)        17, the core part 18 having a crisscross cross section        orthogonal to the axial direction,    -   a plurality of first framework parts 21 formed at regular        intervals along the direction of the shaft core (center axis) 17        in the core part 18 between the first connecting part 15 and the        second connecting part 16, each of the first framework parts 21        having a discoid cross section orthogonal to the axial        direction,    -   a second framework part 22 disposed in a portion partitioned by        the first connecting part 15, the first framework part 21        adjacent to the first connecting part 15, and the core part 18,        the second framework part 22 extending across the first        connecting part 15, the first framework part 21, and the core        part 18 like a diagonal brace,    -   a third framework part 23 disposed in a portion partitioned by a        pair of the first framework parts 21 and 21 adjacent to each        other and the core part 18, the third framework part 23        extending across the pair of first framework parts 21 and 21        adjacent to each other and the core part 18 like a diagonal        brace, and    -   a fourth framework part 24 disposed in a portion partitioned by        the second connecting part 16, the first framework part 21        adjacent to the second connecting part 16, and the core part 18,        the fourth framework part 24 extending across the second        connecting part 16, the first framework part 21, and the core        part 18 like a diagonal brace.

Advantageous Effects of Invention

Even when sudden changes in torque are applied, the injection moldedshaft according to the invention can absorb the energy caused by suddenchanges in torque by undergoing twisting deformation of the shaft bodyand reduce the shock caused by sudden changes in torque using the shaftbody.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating the use state of an injection moldedshaft according to a first embodiment of the invention.

FIG. 2A is a front view (seen from an X-axis direction) illustrating theinjection molded shaft according to the first embodiment of theinvention, FIG. 2B is a left side view illustrating the injection moldedshaft seen from the direction of an arrow B1 in FIG. 2A, FIG. 2C is aright side view illustrating the injection molded shaft seen from thedirection of an arrow B2 in FIG. 2A, FIG. 2D is a cross sectional viewillustrating the injection molded shaft taken along a line A1-A1 in FIG.2A, FIG. 2E is a cross sectional view illustrating the injection moldedshaft taken along a line A2-A2 in FIG. 2A, and FIG. 2F is a crosssectional view illustrating the injection molded shaft taken along aline A3-A3 in FIG. 2A.

FIG. 3A is a plan view (seen from a Y-axis direction) illustrating theinjection molded shaft according to the first embodiment of theinvention, FIG. 3B is a cross sectional view illustrating the injectionmolded shaft taken along the line A1-A1 in FIG. 3A, FIG. 3C is a crosssectional view illustrating the injection molded shaft taken along theline A2-A2 in FIG. 3A, and FIG. 3D is a cross sectional viewillustrating the injection molded shaft taken along the line A3-A3 inFIG. 3A.

FIG. 4 is an enlarged view illustrating a part of the injection moldedshaft in FIG. 2A.

FIG. 5A is a diagram illustrating an injection molded shaft die takenalong a Y-Z coordinate plane and FIG. 5B is a diagram illustrating theinjection molded shaft die taken along an X-Z coordinate plane.

FIG. 6A is a diagram illustrating an injection molded shaft according toa first modification of the first embodiment of the invention and anenlarged view (corresponding to FIG. 4) illustrating a part of a shaftbody. In addition, FIG. 6B is a diagram illustrating an injection moldedshaft according to a second modification of the first embodiment of theinvention and an enlarged view (corresponding to FIG. 4) illustrating apart of a shaft body.

FIG. 7 is a diagram illustrating an injection molded shaft according toa third modification of the first embodiment of the invention and thediagram illustrates one end in an axial direction of the injectionmolded shaft.

FIGS. 8A-8F are diagrams illustrating an injection molded shaftaccording to a fourth modification of the first embodiment of theinvention and the diagrams correspond to FIGS. 2A-2F.

FIG. 9 is a diagram illustrating an injection molded shaft according toa fifth modification of the first embodiment of the invention and thediagram corresponds to FIG. 2A.

FIG. 10 is a vertical cross sectional view illustrating a conventionalinjection molded shaft.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention will be described in detail below withreference to the drawings.

First Embodiment

FIGS. 1 to 4 illustrate the injection molded shaft 1 according to thefirst embodiment of the invention. FIG. 1 is a diagram illustrating theuse state of the injection molded shaft 1. In addition, FIG. 2A is afront view (seen from the X-axis direction) illustrating the injectionmolded shaft 1, FIG. 2B is a left side view illustrating the injectionmolded shaft 1 seen from the direction of the arrow B1 in FIG. 2A, FIG.2C is a right side view illustrating the injection molded shaft 1 seenfrom the direction of the arrow B2 in FIG. 2A, FIG. 2D is a crosssectional view illustrating the injection molded shaft 1 taken along theline A1-A1 in FIG. 2A, FIG. 2E is a cross sectional view illustratingthe injection molded shaft 1 taken along the line A2-A2 in FIG. 2A, andFIG. 2F is a cross sectional view illustrating the injection moldedshaft 1 taken along the line A3-A3 in FIG. 2A. In addition, FIG. 3A is aplan view (seen from the Y-axis direction) illustrating the injectionmolded shaft 1, FIG. 3B is a cross sectional view illustrating theinjection molded shaft 1 taken along the line A1-A1 in FIG. 3A, FIG. 3Cis a cross sectional view illustrating the injection molded shaft 1taken along the line A2-A2 in FIG. 3A, and FIG. 3D is a cross sectionalview illustrating the injection molded shaft 1 taken along the lineA3-A3 in FIG. 3A. In addition, FIG. 4 is an enlarged view illustrating apart of the injection molded shaft 1 (particularly the shaft body 2) inFIG. 2A.

Structure of Injection Molded Shaft

As illustrated in FIGS. 1 to 3, the injection molded shaft 1 includes ahelical gear 3 as the first torque applied part formed at a first end inthe axial direction, a worm 4 as the second torque applied part formedat a second end in the axial direction, and the shaft body 2 connectingthe helical gear 3 to the worm 4 integrally along the shaft coredirection. In addition, the injection molded shaft 1 has a rod-shapedfirst boss 5 formed integrally at the rotation center of a side 3 a ofthe helical gear 3 and a rod-shaped second boss 6 formed integrally atthe rotation center of a side 4 a of the worm 4 (see FIGS. 2A to 2C).The injection molded shaft 1 in such a structure is integrally formed byinjecting molten resin such as POM (polyacetal) or PA (polyamide) into acavity 8 of a die 7 as described in detail later.

As illustrated in FIG. 1, in the injection molded shaft 1, the helicalgear 3 at the first end in the axial direction is engaged with anotherfirst helical gear 10 to form a screw gear 11, and the worm 4 at thesecond end in the axial direction is engaged with another second helicalgear 12 to form a worm gear 13. In the injection molded shaft 1 asdescribed above, when, for example, the rotation of a motor (notillustrated) or the like is transferred via the screw gear 11, therotation is transferred to the second helical gear 12 via the worm 4formed integrally with the shaft body 2. At this time, the shaft body 2of the injection molded shaft 1 undergoes twisting deformation by therotation torque applied via the helical gear 3 at the one end in theaxial direction and the rotation torque applied via the worm 4 at theother end in the axial direction.

As illustrated in FIGS. 2 to 4, in the shaft body 2 of the injectionmolded shaft 1, the discoid first connecting part 15 positioned at thefirst end in the axial direction is formed integrally with the helicalgear 3 and the discoid second connecting part 16 positioned at the otherend in the axial direction is formed integrally with the worm 4. In theshaft body 2, the first connecting part 15 is connected to the secondconnecting part 16 by the core part 18 extending along the shaft core(center axis) 17. The core part 18 has a crisscross cross sectionorthogonal to the axial direction and is positioned so that the centerof the intersecting portion of the cross is aligned with the shaft core(center axis) 17. In the core part 18 between the first connecting part15 and the second connecting part 16, the plurality of first frameworkparts 21 are formed at regular intervals along the direction in whichthe shaft core (center axis) 17 extends. The first framework parts 21are formed into the core part 18 so as to have discoid cross sectionsorthogonal to the axial direction. In the shaft body 2, in the portionpartitioned by the first connecting part 15, the first framework part 21adjacent to the first connecting part 15, and the core part 18, thesecond framework part 22 is disposed. The second framework part 22extends across the first connecting part 15, the first framework part21, and the core part 18 like a diagonal brace and includes a pair ofdiagonal brace members 22 a and 22 b intersecting with each other likean X shape. In the shaft body 2, the third framework part 23 is disposedin the portion partitioned by the pair of first framework parts 21 and21 adjacent to each other and the core part 18. The third framework part23 extends across the pair of first framework parts 21 and 21 and thecore part 18 like a diagonal brace and includes the pair of diagonalbrace members 23 a and 23 b intersecting with each other like an X shapeas in the second framework part 22. In the shaft body 2, the fourthframework part 24 is disposed in the portion partitioned by the secondconnecting part 16, the first framework part 21 adjacent to the secondconnecting part 16, and the core part 18. The fourth framework part 24extends across the second connecting part 16, the first framework part21, and the core part 18 like a diagonal brace and includes a pair ofdiagonal brace members 24 a and 24 b intersecting with each other likean X shape as in the second and third framework parts 22 and 23.

As illustrated in FIGS. 2D and 2E, FIGS. 3B and 3C, and FIG. 4, in theshaft body 2 of the injection molded shaft 1, a first core portion 18 aof the core part 18 extending along the Y-axis has a wall thickness of Wand a second core portion 18 b of the core part 18 extending along theX-axis has a wall thickness of W, which is the same as the wallthickness W of the first core portion 18 a. The wall thickness of thesecond core portion 18 b is measured at the connection portion with thefirst core portion 18 a. In FIGS. 2 to 4, the X-axis matches thedirection in which a first movable mold 33 for molding the shaft body 2is separated (see FIG. 5B) and the second core portion 18 b has adisconnection gradient for facilitating separation from the firstmovable mold 33. As a result, the wall thickness of the second coreportion 18 b becomes small with distance from the first core portion 18a along the X-axis direction. In addition, in the first to fourthframework parts 21 to 24, a wall thickness of a connection portion withthe first core portion 18 a is W, which is the same as the wallthickness W of the first core portion 18 a. The first to fourthframework parts 21 to 24 have a disconnection gradient as in the secondcore portion 18 b and the wall thickness becomes small with distancefrom the first core portion 18 a along the X-axis direction. Inaddition, the shaft body 2 of the injection molded shaft 1 is formed soas to have the same outer dimension (D) from the one end in the axialdirection to the other end in the axial direction. In the shaft body 2of the injection molded shaft 1 described above, the wall thicknesses Wof the core part 18 and the first to fourth framework parts 21 to 24 aredetermined so as to enable flexible twisting deformation as comparedwith a conventional cylindrical shaft (see FIG. 9).

As illustrated in FIGS. 2A and 3A, the shaft body 2 of the injectionmolded shaft 1 is line-symmetric with respect to the shaft core 17 alongthe Z-axis direction and a plurality of constant shapes formed by thefirst to fourth framework parts 21 to 24 and the like are formed atregular intervals along the shaft core (center axis) 17. In addition, asillustrated in FIGS. 2D to 2F and 3B to 3D, the cross section orthogonalto the axial direction of the shaft body 2 of the injection molded shaft1 is line-symmetric with respect to a center line CL1 along the X-axisand a center line CL2 along the Y-axis. Accordingly, the injectionmolded shaft 1 according to the embodiment is accuratelyinjection-molded since the core part 18 and the first to fourthframework parts 21 to 24 of the shaft body 2 are formed to have the samewall thickness W in addition to the above reason.

As illustrated in FIG. 4, in the shaft body 2 of the injection moldedshaft 1, the pairs of diagonal brace members 22 a to 24 a and 22 b to 24b constituting the second to fourth framework parts 22 to 24 intersectwith the shaft core (center axis) 17 of the injection molded shaft 1 atan angle θ. The angle θ is appropriately set depending on the spacingbetween the first connecting part 15 and the first framework part 21adjacent to the first connecting part 15, the spacing between the pairof first framework parts 21 and 21 adjacent to each other, the spacingbetween the second connecting part 16 and the first framework part 21adjacent to the second connecting part 16, the outer dimension D of theshaft body 2, and the like.

Injection Molding Die

FIG. 5 is a diagram schematically illustrating the injection molding die7 for the injection molded shaft 1 according to the embodiment. FIG. 5Ais a cross sectional view illustrating the injection molding die 7 takenalong a Y-Z coordinate plane of the rectangular coordinate system andFIG. 5B is a cross sectional view illustrating the injection molding die7 taken along an X-Z coordinate plane of the rectangular coordinatesystem.

As illustrated in FIG. 5, the injection molding die 7 includes a fixedmold 25 and a movable mold 26. The fixed mold 25 includes a first fixedmold 28 having a first cavity 27 for shaping the first boss 5 at thefirst end in the axial direction of the injection molded shaft 1 and asecond fixed mold 31 having a second cavity 30 for shaping the helicalgear 3 of the injection molded shaft 1. The movable mold 26 includes thefirst movable mold (shaft body formation portion of the injectionmolding die 7) 33 in which a third cavity 32 for shaping the shaft body2 of the injection molded shaft 1 is formed, a second movable mold 35 inwhich a fourth cavity 34 for shaping the worm 4 of the injection moldedshaft 1 is formed, and a third movable mold 37 in which a fifth cavity36 for shaping the second boss 6 at the other end in the axial directionof the injection molded shaft 1 is formed. The first movable mold 33 issplit into two so as to be opened along the X-axis direction from theposition of the shaft core (center axis) 17 of the third cavity 32 (seeFIG. 5B). In addition, a gate 38 is provided in the first fixed mold 28of the injection molding die 7 so as to be opened toward the inside ofthe first cavity 27. In addition, the first to fifth cavities 27, 30,32, 34, and 36 constitute the cavity 8 for shaping the injection moldedshaft 1.

As illustrated in FIG. 5, in the state in which the fixed mold 25 andthe movable mold 26 are closed, molten synthetic resin is injected intothe first cavity 27 from the gate 28 and the molten synthetic resininjected into the first cavity 27 is supplied to the second to fifthcavities 30, 32, 34, and 36. As a result, the entire injection moldedshaft integrally formed therein and described above has a unitary,one-piece construction formed of the injected resin material. When theinjection molding die 7 is configured by changing the fixed mold 25 inFIG. 5 to a movable mold and the movable mold 26 in FIG. 5 to a fixedmold, the gate 28 is provided so as to be opened toward the fifth cavity36 to be positioned on the fixed mold side.

In the state illustrated in FIG. 5, after the synthetic resin injectedinto the cavity 8 of the injection molding die 7 is cooled andsolidified (after the injection molded shaft 1 is formed), the movablemold 26 is separated from the fixed mold 25 while being rotated (movedin the Z-axis direction). This causes the injection molded shaft 1 to beseparated from the fixed mold 25 while being held by the movable mold26. Next, the first movable mold 33 is opened (split into two) along theX-axis direction, the first boss 5, the helical gear 3, and the shaftbody 2 are exposed from the second movable mold 35, the worm 4 is housedin the second movable mold 35, and the second boss 6 is housed in thethird movable mold 37. Next, an eject pin 40 housed in the third movablemold 37 in a slidable state continuously presses the second boss 6 ofthe injection molded shaft 1, the worm 4 moves in the second movablemold 35 while being rotated, the worm 4 is pushed out of the secondmovable mold 35, and the injection molded shaft 1 is removed from theinjection molding die 7. Note that the first movable mold 33 of theinjection molding die 7 is opened (split into two) along the Y-axisdirection in the coordinate axes (the up-down direction in FIG. 2D isthe X-axis direction and the left-right direction in FIG. 3B is theX-axis direction) obtained by rotating the cross sections orthogonal tothe axial direction illustrated in FIGS. 2D and 3B counterclockwise(left rotation direction) by 90 degrees.

Effect of Present Embodiment

The injection molded shaft 1 according to the embodiment as describedabove can absorb the energy caused by sudden changes in torque byundergoing the flexible twisting deformation of the shaft body 2 evenwhen sudden changes in torque are applied, and can reduce the shockcaused by sudden changes in torque by undergoing the twistingdeformation of the shaft body 2. As a result, the injection molded shaft1 according to the embodiment can prevent the helical gear 3 formed atthe one end in the axial direction and the worm 4 formed at the otherend in the axial direction from receiving an excess load, prevent theteeth of the helical gear 3 at the one end in the axial direction andthe teeth of another first helical gear 10 engaged with the helical gear3 from being broken, and prevent the teeth of the worm 4 formed at theother end in the axial direction and the teeth of another second helicalgear 12 engaged with the worm 4 from being broken.

In addition, since the core part 18 and the first to fourth frameworkparts 21 to 24 of the shaft body 2 in the injection molded shaft 1according to the embodiment have the same wall thickness (W), the shaftbody 2 can be injection-molded accurately without occurrence of amolding failure due to variations in the shrinkage ratio.

In addition, in the injection molded shaft 1 according to theembodiment, many hollowed concave parts 41 to 43 are formed between thefirst connecting part 15, the second framework part 22, the firstframework part 21, and the core part 18. In addition, in the injectionmolded shaft 1, many hollowed concave parts 41 to 43 are formed betweenthe first framework parts 21 and 21 adjacent to each other, the thirdframework part 23, and the core part 18. In addition, in the injectionmolded shaft 1, many hollowed concave parts 41 to 43 are formed betweenthe second connecting part 16, the fourth framework part 24, and thecore part 18. Accordingly, in the injection molded shaft 1 according tothe embodiment, as compared with the case in which the injection moldedshaft 1 is shaped in a rod, since the amount of synthetic resin materialcan be reduced and the cooling time after injection into the cavity 8 ofthe injection molding die 7 can be shortened, the injection moldingcycle can be shortened, the production efficiency can be improved, andthe total weight can be reduced.

In addition, since the injection molded shaft 1 according to theembodiment can reduce a shock caused by sudden changes in torque byundergoing the twisting deformation of the shaft body 2, vibrationscaused by sudden changes in torque can be reduced and the generation ofnoise caused by sudden changes in torque can be suppressed. Accordingly,the injection molded shaft 1 according to the embodiment reducesoperation noise during power transmission.

In addition, in the injection molded shaft 1 according to theembodiment, since the plurality of hollowed concave parts 43 are formedat regular intervals along the axial direction at the end of the corepart 18 (the first core portion 18 a and the second core portion 18 b)of the shaft body 2, voids (air bubbles) are not easily generated,thereby efficiently preventing occurrence of a molding failure caused byvoids.

First Modification and Second Modification of First Embodiment

FIG. 6A is a diagram illustrating an injection molded shaft 1 accordingto a first modification of the first embodiment of the invention and anenlarged view (corresponding to FIG. 4) illustrating a part of the shaftbody 2. In addition, FIG. 6B is a diagram illustrating an injectionmolded shaft 1 according to a second modification of the firstembodiment of the invention and an enlarged view (corresponding to FIG.4) illustrating a part of the shaft body 2.

In the injection molded shaft 1 according to the first embodiment of theinvention, the second to fourth framework parts 22 to 24 include thepairs of diagonal brace members 22 a to 24 a and 22 b to 24 bintersecting with each other like an X shape. However, the invention isnot limited to the injection molded shaft 1 according to the firstembodiment and each of the second to fourth framework parts 22 to 24 maybe configured by one diagonal brace member, which is one of 22 a to 24 a(or 22 b to 24 b), and the twisting stiffness of the injection moldedshaft 1 may be reduced according to the use condition and the like. Inthe injection molded shaft 1 illustrated in FIGS. 6A and 6B, the samestructural portions as in the injection molded shaft 1 illustrated inFIG. 4 are given the same reference numerals and duplicate descriptionsas in the injection molded shaft 1 according to the first embodiment areomitted.

Third Modification of First Embodiment

FIG. 7 is a diagram illustrating an injection molded shaft 1 accordingto a third modification of the first embodiment of the invention and adiagram illustrating one end in the axial direction of the injectionmolded shaft 1.

As illustrated in FIG. 7, in the injection molded shaft 1 according tothe first embodiment of the invention, when the outer dimension of thefirst boss 5 close to the helical gear 3 is large, a hollowed hole 44 isdesirably formed along the center axis 17 from an end face 5 a of thefirst boss 5 to the first connecting part 15 of the shaft body 2. In theinjection molded shaft 1 in which this hollowed hole 44 is formed,occurrence of a molding failure due to shrinkage or voids can beprevented and the cooling time and the injection molding cycle can beshortened. Although not illustrated in the drawing, in the injectionmolded shaft 1 according to the first embodiment, when the outerdimension of the first boss 6 close to the worm 4 is large, to obtainthe same effect of the hollowed hole 44 in the first boss 5, a hollowedhole is desirably formed along the center axis 17 from the end face ofthe second boss to the second connecting part 16 of the shaft body 2(see FIG. 2).

Fourth Modification of First Embodiment

FIGS. 8A-8F are diagrams illustrating an injection molded shaft 1according to a fourth modification of the first embodiment of theinvention and the diagrams correspond to FIGS. 2A-2F. In the injectionmolded shaft 1 illustrated in FIGS. 8A-8F, the same structural portionsas in the injection molded shaft 1 illustrated in FIG. 2 are given thesame reference numerals and duplicate descriptions as in the injectionmolded shaft 1 according to the first embodiment are omitted.

As illustrate in FIGS. 8A-8F, the injection molded shaft 1 according tothe modification has a semicylindrical hollowed concave part 46 at anintersecting portion (first intersecting portion) 45 at which the secondframework part 22 (diagonal brace members 22 b) positioned at the oneend in the axial direction of the shaft body 2 intersects with the corepart 18. When seen from the direction along the X-axis, the chord ofthis semicylindrical hollowed concave part 46 is positioned along theouter periphery of the border between the first connecting part 15 andthe intersecting portion 45. In addition, the injection molded shaft 1has a cylindrical hollowed concave part 48 at an intersecting portion(second intersecting portion) 47 between the first framework part 21,the second framework part 22 (diagonal brace members 22 a), the thirdframework part 23 (diagonal brace members 23 b), and the core part 18.In addition, the injection molded shaft 1 has a cylindrical hollowedconcave part 51 at an intersecting portion (third intersecting portion)50 between the first framework part 21, the third framework part 23(diagonal brace members 23 a and 23 b), and the core part 18. Inaddition, the injection molded shaft 1 has a cylindrical hollowedconcave part 53 at an intersecting portion (fourth intersecting portion)52 between the first framework part 21, the third framework part 23(diagonal brace members 23 a), the fourth framework part 24 (diagonalbrace members 24 b), and the core part 18. In addition, the injectionmolded shaft 1 has a semicylindrical hollowed concave part 55 at anintersecting portion (fifth intersecting portion) 54 between the fourthframework part 24 (diagonal brace members 24 a) and the core part 18.When seen from the direction along the X-axis, the chord of thesemicylindrical hollowed concave part 55 is positioned along the outerperiphery of the border between the second connecting part 16 and theintersecting portion 54.

The above hollowed concave parts 46, 48, 51, 53, and 55 areline-symmetric with respect to the center line CL2 along the Y-axis (seeFIGS. 8A and 8F). In addition, each of the hollowed concave parts 46,48, 51, 53, and 55 has a disconnection gradient for facilitatingseparation from the injection molding die 7 and the wall thicknessbetween these hollowed concave parts and the adjacent hollowed concaveparts 41 and 42 is substantially the same as the wall thickness W of theconnection portion between the first to fourth framework parts 21 to 24and the first core portion 18 a. In addition, the hollowed concave parts46, 48, 51, 53, and 55 have a depth that reaches the first core portion18 a (see FIGS. 8A and 8F). The invention is not limited to the case inwhich the wall thickness between the hollowed concave parts 46, 48, 51,53, and 55 and the adjacent hollowed concave parts 41 and 42 issubstantially the same as the wall thickness W of the connection portionbetween the first to fourth framework parts 21 to 24 and the first coreportion 18 a and the wall thickness may be changed depending on the sizeof the shaft body 2, the magnitude of transfer torque, and the like. Inaddition, the depths of the hollowed concave parts 46, 48, 51, 53, and55 are not limited to the depths that reach the first core portion 18 aand the depths may be changed depending on the size of the shaft body 2,the magnitude of transfer torque, and the like.

In addition, the hollowed concave parts 46, 48, 51, 53, and 55 areformed orthogonally to the Y-Z coordinate plane and opened along theopen direction of the injection molding die 7 (see FIG. 5).

In the injection molded shaft 1 according to the modification asdescribed above, since the hollowed concave parts 46, 48, 51, 53, and 55are formed, the number of portions having substantially the same wallthickness is larger than in the injection molded shaft 1 according tothe first embodiment. Accordingly, the accuracy of the shape afterinjection molding is higher than in the injection molded shaft 1according to the first embodiment.

Fifth Modification of First Embodiment

FIG. 9 is a front view of an injection molded shaft 1 according to themodification and the front view corresponds to FIG. 2A. As illustratedin FIG. 9, in the injection molded shaft 1 according to themodification, the shaft body 2 is shorter than the shaft body 2 of theinjection molded shaft 1 according to the first embodiment. That is, inthe modification, the shaft body 2 of the injection molded shaft 1includes the discoid first connecting part 15 formed integrally with thefirst torque applied part 3, the discoid second connecting part 16formed integrally with the second torque applied part 4, the core part18, extending from the first connecting part 15 to the second connectingpart 16 along the shaft core 17, that has a crisscross cross sectionorthogonal to the axial direction, and a diagonal-brace-shaped frameworkpart 60, disposed in the portion partitioned by the first connectingpart 15, the second connecting part 16, and the core part 18, thatextends across the first connecting part 15, the second connecting part16, and the core part 18 like a diagonal brace. Thediagonal-brace-shaped framework part 60 includes the pair of diagonalbrace members 22 a and 22 b intersecting like an X shape or the pair ofdiagonal brace members 24 a and 24 b intersecting like an X shape asdescribed in detail in the first embodiment. In the injection moldedshaft 1 illustrated in FIG. 9, the same structural portions as in theinjection molded shaft 1 illustrated in FIG. 2A are given the samereference numerals and duplicate descriptions as in the first embodimentare omitted.

It can be considered that the shaft body 2 of the injection molded shaft1 as described above is configured by one framework unit 61. That is,the framework unit 61 includes the core part 18, extending along theshaft core 17, that has a crisscross cross section orthogonal to theaxial direction, a pair of discoid framework parts 62 and 62 (the firstconnecting part 15 and the second connecting part 16), disposed at oneend and the other end along the shaft core 17 of the core part 18 so asto face each other, that have discoid cross sections orthogonal to theaxial direction, the diagonal-brace-shaped framework parts 60 and 60,disposed in the portion partitioned by the pair of discoid frameworkparts 62 and 62 and the core part 18, that extend across the pair ofdiscoid framework parts 62 and 62 and the core part 18 like a diagonalbrace.

In the injection molded shaft 1 according to the modification asdescribed above, even when sudden changes in torque are applied, theenergy caused by sudden changes in torque can be absorbed by flexibletwisting deformation of the shaft body 2 and the shock caused by suddenchanges in torque can be reduced by twisting deformation of the shaftbody 2.

When the injection molded shaft 1 according to the first embodiment isconsidered from the viewpoint of configuring the shaft body 2 using theframework unit 61 like the injection molded shaft 1 according to themodification, the shaft body 2 of the injection molded shaft 1 accordingto the first embodiment can be considered to have a plurality of (six)framework units 61. In the shaft body 2 of the injection molded shaft 1according to the first embodiment, the first connecting part 15, thesecond connecting part 16, and the first framework part 21 areequivalent to the discoid framework parts 62. In addition, in the shaftbody 2 of the injection molded shaft 1 according to the firstembodiment, the pair of diagonal brace members 22 a and 22 bintersecting like an X shape, the pair of diagonal brace members 23 aand 23 b intersecting like an X shape, and the pair of diagonal bracemembers 24 a and 24 b are equivalent to the diagonal-brace-shapedframework part 60. In addition, in the injection molded shaft 1, theshaft body 2 may be configured by two or more pairs of framework units61.

In addition, in the injection molded shaft 1 according to themodification, the diagonal-brace-shaped framework part 60 is configuredby the pair of diagonal brace members 22 a and 22 b intersecting like anX shape or the pair of diagonal brace members 24 a and 24 b intersectinglike an X shape. However, the invention is not limited to thismodification and the diagonal-brace-shaped framework part 60 may beconfigured by one diagonal brace member 22 a (24 a) or one diagonalbrace member 22 b (24 b).

Other Modifications

The injection molded shaft 1 according to the invention is not limitedto the first embodiment described above and the first torque appliedpart may be a gear other than a helical gear, such as a spur gear orbevel gear and the second torque applied part may be a gear other than aworm, such as a spur gear or bevel gear. In addition, in the injectionmolded shaft 1 according to the invention, the first torque applied partand the second torque applied part only need to be portions to which arotation torque is applied and may be, for example, a spline formationpart, a key groove formation part, or the like for fixing a helical gearor the like.

REFERENCE SIGNS LIST

-   1: injection molded shaft-   2: shaft body-   3: helical gear (first torque applied part)-   4: worm (second torque applied part)-   15: first connecting part-   16: second connecting part-   17: shaft core (longitudinal center axis)-   18: core part-   21: first framework part-   22: second framework part-   23: third framework part-   24: fourth framework part-   33: first movable mold (shaft body formation portion)-   60: diagonal-brace-shaped framework part-   61: framework unit-   62: discoid framework part

The invention claimed is:
 1. An injection molded shaft comprising: afirst torque applied part formed at a first end in an axial direction; asecond torque applied part formed at a second end in the axialdirection; and a shaft body connecting the first torque applied part tothe second torque applied part along a direction of a longitudinalcenter axis of the injection molded shaft, wherein the shaft bodyincludes: a first connecting part formed integrally with the firsttorque applied part, a second connecting part formed integrally with thesecond torque applied part, a core part extending from the firstconnecting part to the second connecting part along the longitudinalcenter axis, the core part having a crisscross cross section orthogonalto the axial direction, a plurality of first framework parts formed atregular intervals along the direction of the longitudinal center axis inthe core part between the first connecting part and the secondconnecting part, each of the first framework parts having a discoidcross section orthogonal to the axial direction, a second framework partdisposed in a portion partitioned by the first connecting part, thefirst framework part adjacent to the first connecting part and the corepart, the second framework part being a diagonal brace extending acrossthe first connecting part, the first framework part, and the core part,a third framework part disposed in a portion partitioned by a pair ofthe first framework parts adjacent to each other and the core part, thethird framework part being a diagonal brace extending across the pair offirst framework parts and the core part, a fourth framework partdisposed in a portion partitioned by the second connecting part, thefirst framework part adjacent to the second connecting part and the corepart, the fourth framework part being a diagonal brace extending acrossthe second connecting part, the first framework part, and the core part,a first intersecting portion between the second framework part and thecore part, a second intersecting portion between the first frameworkpart, the second framework part, the third framework part, and the corepart, a third intersecting portion between the first framework part, thethird framework part, and the core part, a fourth intersecting portionbetween the first framework part, the third framework part, the fourthframework part, and the core part, and a fifth intersecting portionbetween the fourth framework part and the core part, and wherein each ofthe first intersecting portion to the fifth intersecting portion has ahollowed concave part opened toward a die opening direction of aninjection molding die.
 2. The injection molded shaft according to claim1, wherein the hollowed concave part of each of the first intersectingportion to the fifth intersecting portion has a depth reaching the corepart.
 3. The injection molded shaft according to claim 1, wherein thehollowed concave part of each of the first intersecting portion to thefifth intersecting portion has a disconnection gradient configured tofacilitate separation of the injection molded shaft from the injectionmolding die.
 4. The injection molded shaft according to claim 1, whereinthe hollowed concave part of the first intersecting portion and thefifth intersecting portion are semicylindrical, and the hollowed concavepart of the second intersecting portion, the third intersecting portion,and the fourth intersecting portion are cylindrical.